1. Field of the Invention
The present invention relates to a printer with a micromirror device as a spatial light modulator and an exposure method therefor, by which a hard copy of an image is reproduced with high-fidelity without lowering print speed.
2. Background Arts
A spatial light modulator (SLM) has been used for deflecting a light beam, for example, in a laser optical system. Recently, a mirror type spatial light modulator has been developed which has an array of very small mirror elements called micromirrors, whose tilt angle is each individually changeable to control the deflection. As the mirror type SLM, there are a digital micromirror devices (DMD) which use electrostatic forces to move the micromirror, and piezoelectric type micromirror devices (AMA) which move the micromirror by the use of mechanical deformation of fine piezoelectric elements. Because the micromirror devices have an imaging function, their applications to image projectors and printers has been studied. The principle and applications of the DMD are described, for example, in a monthly magazine O plus E, October 1994, pages 90-94.
For example, a DMD has an array of large number of micromirrors, and a static RAM (SRAM) which are integrally formed on a silicone substrate by a semiconductor technique. One micromirror of the array is mounted above each memory cell of the SRAM, so as to sway about a diagonal axis. While the power is off, the micromirrors are oriented horizontal or parallel to the plane of the substrate, hereinafter called a balanced state. When 1-bit mirror drive data is written in each memory cell, the corresponding micromirror tilts by an angle +.theta. or -.theta. from the balanced state, depending upon the binary value of the mirror drive data. For example, the position tilt by the angle +.theta. is an ON-state where the micromirror reflects illumination light projected diagonally from a light source, e.g. an angle 2.theta. from the perpendicular to the substrate plane, toward an imaging lens. A light spot or an image of the micromirror is formed through the imaging lens onto a photographic material or a screen, constituting a pixel. The position tilt by the angle -.theta. is an OFF-state where the illumination light reflected from the micromirror is directed to a light absorbing plate, and is not utilized for imaging.
Since the DMD operates in a digital fashion, the printer or hardcopy system with the DMD controls the total exposure time per pixel by changing the total time duration of the ON-state of the micromirror per pixel, to print a gradation image. That is, the tonal level of one pixel is controlled by changing the time duration of a continuous ON-state and/or the number of intermittent ON-states of the micromirror per pixel. The time durations of the ON- or OFF-state of the micromirror can be controlled according to a conventional pulse width modulation method. For example, it is possible to reduce the driving time intervals of the micromirrors half by half within an exposure sequence for one line. However, the minimum time duration and the minimum interval of ON-OFF switching are limited by the time necessary for the micromirror to switch over between the ON-state and the OFF-state. The shortest possible mirror response time is about 15 .mu.seconds in the state of art.
For example, in order to achieve 256 tonal levels per pixel at a print density of 600 dpi (dot/inch) and at a print speed of 1.75 inch/sec by use of a 8-bit pulse width modulation, the DMD micromirror is required to switch over in a very short time of about 1 .mu.seconds. Since the mirror response time is about 15 .mu.seconds, the printer with the DMD cannot achieve such a fine gradation as above. JPA 7-131648 discloses a gradation printing method which is made to solve the above problem in the gradation printing with a spatial light modulator.
According to the Japanese prior art, a micromirror array consists of a lot of rows of micromirrors to perform multiple-exposure or row integration, so the response speed of the DMD may be relatively low, that is, the response time of the DMD can be relatively long. For example, using a micromirror array consisting of 64 rows, the necessary response time of the DMD is about 60 .mu.seconds for the above 256 tonal level printing. Therefore, the 256 tonal levels is possible according to this method.
However, since the above prior method requires a large number of micromirrors, and needs more micromirrors to achieve more tonal levels or higher print speed, the cost is inevitably high. Moreover, for color printing, the response speed of the micromirrors are required to be three times as high as that required for monochrome printing in order to achieve the same tonal levels at the same print speed. Therefore even with the micromirror array having 64 rows, the response time should be less than 20 .mu.seconds to achieve the 256 tonal levels.